The Mechanism of a Bacterial Transposition 
Reaction 
Nancy L. Craig, Ph.D. — Associate Investigator 
Dr. Craig is also Associate Professor of Molecular Biology and Genetics at the Johns Hopkins University 
School of Medicine. After receiving a bachelor's degree in biology and chemistry from Bryn Mawr College, 
she did graduate work on bacterial responses to DMA damage with Jeffrey Roberts at Cornell University, 
leading to her Ph.D. degree in biochemistry. She did postdoctoral research on the integration/excision 
cycle of the bacteriophage X with Howard Nash at the National Institute of Mental Health. Before joining 
HHMI, she was Associate Professor of Microbiology and Immunology and of Biochemistry and Biophysics 
at the University of California, San Francisco. 
DESPITE DNA's essential role in maintaining 
accurate genetic information, this molecule 
displays a surprising degree of plasticity. We now 
know that DNA rearrangements — i.e., the reorga- 
nization of DNA sequences through breakage, 
translocation, and rejoining reactions — mediate 
a wide variety of fundamental cellular processes. 
DNA rearrangements play an important role in the 
control of gene expression during development, 
the acquisition of new genetic elements such as 
viruses, the repair of damaged DNA, and the cre- 
ation of genetic diversity. We are interested in 
understanding at the molecular level how DNA 
rearrangements occur and are controlled. 
We are particularly interested in the type of 
recombination called transposition. In this reac- 
tion, a discrete DNA segment moves from one po- 
sition in a genome into another, nonhomologous 
target position. This translocation may occur be- 
tween different positions on the same chromo- 
some, between chromosomes, or between chro- 
mosomes and extrachromosomal elements such 
as plasmids. Transposable elements have been 
identified in a wide variety of organisms. A nota- 
ble consequence of transposition is the promo- 
tion of rapid and extensive genetic change. Trans- 
poson insertion results in the stable linkage of 
information encoded by the transposon with the 
target DNA. Transposon insertion into a gene 
will likely inactivate the gene, and insertion into 
DNA sequences that control the expression of 
nearby genes may inactivate or activate those 
genes. Probably because of its potential for 
profound influence, transposition is highly 
regulated. 
Our research is focused on understanding the 
transposition of Tn7, a bacterial transposon with 
several unusual properties. Of particular note is 
Tn7's unusual target selectivity. Most transpos- 
able elements display little insertion-site selectiv- 
ity, inserting into many different targets. By con- 
trast, Tn7 inserts at high frequency into a single, 
specific site in the chromosomes of many bacte- 
ria. In Escherichia colt, the organism in which 
we study Tn7, this special target is called an at- 
tachment site and designated attTnl. When 
attTn 7 is unavailable, Tn7 resembles most other 
transposable elements, inserting at low fre- 
quency into many target sites. 
Like most mobile DNA segments, Tn7 encodes 
the machinery that mediates its transposition 
from place to place. We have established that Tn7 
encodes a surprisingly complex array of transposi- 
tion proteins, and we have also identified the par- 
ticular DNA sequences at the ends of Tn7 and at 
its insertion sites that are the actual substrates for 
transposition. The high-frequency insertion of 
Tn7 into attTnl is mediated by four Tn7- 
encoded genes, tnsABC + tnsD\ Tn7 insertion 
into other random target sites, a low-frequency 
reaction, is mediated by a distinct set of Tn7-en- 
coded genes, tnsABC + tnsE. Tn7 also encodes 
resistance to the antibiotics trimethoprim and 
streptomycin/spectinomycin. The ability of Tn7 
and other mobile DNA segments that encode anti- 
biotic resistance to insert into and thus be joined 
to plasmids that can, in turn, move among a vari- 
ety of bacterial species underlies the rapid dis- 
semination of antibiotic resistance among bacte- 
rial populations. 
Our overall goals are to understand in molecu- 
lar detail how Tn7 moves from place to place 
and how the frequency of this movement is 
modulated. We expect that understanding the 
macromolecular interactions that underlie Tn7 
transposition will contribute not only to the 
understanding of DNA recombination but also 
to the understanding of other complex protein- 
nucleic acid transactions such as DNA replica- 
tion, transcription, and RNA processing. We are 
using a variety of biochemical and genetic ap- 
proaches to dissect Tn7 transposition. 
The fundamental steps in transposition are the 
DNA cleavages that separate the transposon from 
its flanking donor DNA and the subsequent break- 
age of the target DNA and the joining of the trans- 
poson to the target DNA. Little is known in molec- 
ular terms about how such reactions occur. Our 
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